Sulfate attack mechanism

Fig. 6.63 Appearance of normal features not subjected to sulphate attack A—unhydrated cement, B—dense inner C-S-H gel surrounding unhydrated cement, C—inner C-S-H gel constituting fully hydrated cement grain, D— region of small hollow shell hydration grains, E—groundmass or outer product C-S-H gel, F—Ca(OH)j surrounding a sand grain chip, G—deposit of calcium hydroxide within the groundmass (After [261]) Diamond S., Lee R.J. in Materials Science and Concrete, Special volume Sulfate Attack Mechanisms (J. Marchand amd J. Skalny eds.), p. 138, Fig. 1, 1999, published by The American Ceramic Society, 735 Ceramic Place, Westerville, Ohio 43081, 2001, reproduced with the permission of The American Ceramic Society...

Brown, P.W., Taylor, H.F.W. In Marchand, J., Skalny, J.P (eds.) The Role of Ettringite in External Sulphate Attack, p. 73 in Materials Science of Concrete, Sulfate Attack Mechanisms. The American Ceramic Society, Westerville (1999)... 

Even though the mechanisms are not fully understood, it has been fairly well established that SO2 accelerates the corrosion of carbon steel (14-16,49-51). This results in the creation of a layer of rust, i.e. iron oxides, on the surface of the steel, which occupies more than twice the volume of the iron from which it was produced. In addition, Haynie and Upham (15) report that iron oxides catalyze the oxidation of SO2 to SO3 as well as react with SO2 to form sulfates. Both conditions, i.e. the expansion of reinforcing steel due to corrosion and sulfate attack, have been shown to cause the deterioration of PCC. 

Portland-slag cements tend to exhibit improved resistance to sulfate attack compared with plain Portland cement at high slag contents (60% and more), whereas the resistance is scarcely affected or even made worse at low substitution levels (below 25%) (Mehta, 1992 Gollop and Taylor, 1992-1996 Giergiczny, 1997 Krizan and Zivanovic, 1997 Sersale et al, 1997). The improved response to sulfate attack appears to be due mainly to a dilution effect, rather than to aity special mechanism. An increase of the slag content at the expense of clinker results in a lower amount of C3A in the system, which is the phase mainly responsible for the susceptibility of Portland cement to sulfate attack. 

The mechanism of the sulfate attack on Portland-slag cement is broadly similar to that observed in pure Portland cement pastes. In an attack by alkali sulfates the dominant reactions are the partial decalcification of the C-S-H phase and the conversion of the existing monosulfate to ettringite. The amount of ettringite formed is lower than in the case of Portland cement, as monosulfate is the only phase that can serve as a direct source for ettringite formation, whereas neither Al incorporated in the C-S-H phase nor that present in hydrotalcite are available for such a reaction, jnst like Al in ettringite already formed independently of snUate attack (Gollop and Taylor, 1992-1996). 

The "salt weathering" on concrete was just received a lot of attention in the recent years. Based on the above analysis of a limited number of research reports, the experimental results already showed convincing appearances that were completely opposite to the basic principles of salt weathering distress on porous materials. On the contrary, the experimental results of long term field tests and indoor tests rather tended to indicate that chemical sulfate attack is the mechanism for the concrete damage. 

In summary, concerning the role of mineral additions in the sulfate attack on partially exposed concrete, the exposure conditions and the solution transport mechanism are different from the full immersion cases. It needs further research. 

Santhanam Manu, Cohen Menashi D, Olek Jan. Mechanism of sulfate attack afresh look... 

Zanqun Liu, Study of the basic mechanisms of sulfate attack on cementitious materials,... 

A good example of combining information from polished sections in the SEM is the study of the mechanism of sulfate attack made by Yu et al. (2013). Here EDS maps of Ca and S were used to determine sulfate and calcium profiles inside mortars immersed in sodium sulfate solutions. These were used to identify regions of interest, which were studied in detail by BSE images and carefully selected EDS analyses plotted in scatter plots. 

A direct attack on the oxazirane ring occurs with ferrous salts. Oxaziranes are decomposed by aqueous solutions of ferrous ammonium sulfate even at room temperature. The reaction follows a radical-chain mechanism because le.ss than stoichiometric amounts of the ferrous salt cause the decomposition, 2-icrt-Butyl-3-phenyloxazirane (9) and 1 equivalent of ferrous salt give ferf-butylbenzamide in 98%... 

Light, sandy, well-drained soil of high electrical resistivity is low in corrosivity and coated steel or bare stainless steels can be employed. It is unlikely that the whole pipe run would be in the same type of soil. In heavier or damp soils, or where the quality of back filling cannot be guaranteed, there are two major corrosion risks. Steel, copper alloys and most stainless steels are susceptible to sulfide attack brought about by the action of sulfate-reducing bacteria in the soil. SRB are ubiquitous but thrive particularly well in the anaerobic conditions which persist in compacted soil, especially clay. The mechanism of corrosion where SRB are involved is described in Section... 

Sulfation in most aspects is very similar to phosphorylation, except that sulfation is not involved in intracellular signal transduction, but in other forms of signaling. The mechanism of sulfation is similar to that of phosphorylation as a general base from the enzyme active site that deprotonates the hydroxyl groups of tyrosine residues. The nucleophilic oxygen then attacks the /3-position, in contrast to the 7-position in phosphorylation, and releases adenosine 3, 5 -diphosphate. 

Scheme 4 Catalytic mechanism of protein sulfation. Tyrosine attacks electrophilic sulfur generating 3, 5 -ADP as a leaving group.

When a second sulfite or SO2, if you like, or H2SO3 attacks 03Cr0S02 2, it goes all in one fell-swoop to chromic ion, to which sulfate is attached at the end of the reaction, and an SCV - radical. One can t precipitate sulfate with barium immediately after the reaction. Coordination of the chromium (111) stabilizes the radical until it can react with another as was mentioned by Dr. Halpern. This mechanism accounts for both the stoichiometry and kinetics. 

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